DomainKeys Identified Mail (DKIM)
DKIM or DomainKeys Identified Mail (DKIM) is a protocol that uses encryption to verify the sender of an email address. This protocol helps your recipients know that the email is really coming from your servers, or servers you authorized to send on your behalf, and not from people pretending to be you.
Below you will find 2 records that need to be added to your DNS. This is accomplised by adding the record to your DNS Zone Files or though a tool provided by your hosting company.
The private Key as seen above should be added to your email software, their should be a spot to add it. If there isn't, contact that maker of your email software asking them if they support DKIM.